1. Field of the Invention
This invention relates to a push-button switch for use with various input devices, such as a personal computer or a word processor, and more particularly to improvements in or relating to that part of such a switch, the pressing piece, which presses against a movable contact element in the switch element of such a push button switch.
2. Description of the Prior Art
An example of a conventional push-button switch is illustrated in FIGS. 5 to 10. Referring to FIGS. 5 and 6, the conventional push-button switch shown includes a lower case 1 and an upper case 2 which are united with each other by a snap engaging mechanism (not shown) to form a unitary hollow casing. A fitting hole 2a is formed at a substantially central location on the upper wall of the upper case 2, and a stem 3 extends through the fitting hole 2a. A compression coil spring 5 is interposed between the stem 3 and an inner face of a bottom wall of the lower case 1, and a key top 4 is fitted on and secured to the top end of the stem 3. A switch element 6 is disposed at a side location within the casing formed by the lower case 1 and the upper case 2. The switch element 6 is supported on the upper case 2 in such a manner that it is prevented from moving toward the stem 3 by a pair of opposing ribs 2b formed on the opposite inner faces of a pair of side walls of the upper case 2. A pressing spring 11 is fitted at the top end of the switch element 6. The pressing spring 11 has a bent portion 11a, an extension 11b, and a semispherical acting projection 11c. The extension 11b of the compression spring 11 extends below the stem 3 and is bifurcated at its lower end. The extension 11b is located such that its bifurcated portion may move into and out of the locus of reciprocal motion of the stem 3. A leaf spring 12 is disposed on an inner face of a side wall of the upper case 2 in an opposing relationship to the switch element 6 with respect to the stem 3. The leaf spring 12 has an upright portion 12a extending along the inner face of the side wall of the upper case 2, and an extension 12b extending below the stem 3. A lower end portion of the extension 12b of the leaf spring 12 is resiliently engaged with the lower end of the stem 3, and the engaging lower end portion of the extension 12b has the form of a click projection 12c which can be moved into and out of the locus of reciprocal movement of the stem 3 at an intermediate location of its reciprocal movement. The upright portion 12a of the leaf spring 12 is held between the side wall of the upper case 2 and a base portion holding plate 13. The base portion holding plate 13 is supported on the upper case 2 in such a manner that it is prevented from moving toward the stem 3 by a pair of opposing ribs 2c formed on the inner faces of the side walls of the upper case 2.
Referring particularly to FIG. 7, the switch element 6 includes an operating body 7 made of a synthetic resin material, a movable contact element 8 made of a metal material, a spacer 9 made of a synthetic resin material, and a fixed contact element 10 made of a metal plate. The operating body 7 has a pressing piece 7a. The pressing plate 7a is comprised of a leg portion 20, and a movable contact element pressing portion 21. The movable contact element pressing portion 21 of the pressing piece 7a has a greater thickness of material than the leg portion 20 of the pressing piece 7a. As shown in FIGS. 9(a) and 9(b), a rib 22 is formed on one face of the movable contact element pressing portion 21. Adjacent to the operating body 7 is a movable contact element 8. The movable contact element 8 is composed of a frame 8a made of brass, and a resilient thin metal plate 8b made of phosphor bronze or a like material and supported on the frame 8a. Though not shown, gold is plated on one face of the thin metal plate 8b. Adjacent to the movable contact element 8 is a spacer 9. A circular opening 9a is perforated in the spacer 9. Adjacent to the spacer 9 is a fixed contact element 10 made of brass. On the fixed contact element 10 is a gold fixed contact element 10a. The gold fixed contact element 10a is opposed via the opening 9a in the spacer 9 to the thin metal plate 8b of the movable contact element 8. Pairs of engaging holes 8c, 9b and 10b are formed in a mutually aligned relationship at peripheral portions of the movable contact element 8, spacer 9 and fixed contact element 10, respectively. A pair of engaging projections not shown are formed on a rear face of the operating body 7 of the switch element 6 and extend through the engaging holes 8c, 9b and 10b. The ends of the engaging projections are caulked so that the components of the switch element 6, that is, the operating body 7, movable contact element 8, spacer 9 and fixed contact element 10, are united with each other in a layered condition. Note that the movable contact element 8 has a movable side terminal 8d formed thereon while the fixed contact element 10 has a fixed side terminal 10c formed thereon.
The push-button switch described above operates as follows.
When the key top 4 remains in a non-depressed state, as seen in FIG. 5, the stem 3 is positioned at its upper position under the biasing forces of the compression coil spring 5, the pressing spring 11, and the leaf spring 12. In this configuration, the acting projection 11c of the pressing spring 11 does not press against the movable contact element pressing portion 21 of the pressing piece 7a of the operating body 7 of the switch element 6. As a result, the thin metal plate 8b of the movable contact element 8 remains spaced away from the fixed contact 10a of the fixed contact element 10. Accordingly, the push-button switch assumes an off position.
If the key top 4 is depressed, the stem 3 is moved down against the spring 5, and in the course of such downward movement of the stem 3, the click projection 12c of the leaf spring 12 is retracted from the locus of movement of the stem 3. Consequently, a click feeling is provided. Further, since the pressing spring 11 is retracted from, the locus of movement of the stem 3 upon such downward movement of the stem 3, the movable contact element pressing portion 21 of the pressing piece 7a of the operating body 7 is pressed against the thin metal plate 8b of the movable contact element 8 by the acting projection 11c of the pressing spring 11 so that the thin metal plate 8b of the movable contact element 8 is displaced into contact with the fixed contact 10a of the fixed contact element 10, thereby bringing the push-button switch into an on state.
If the depressing force to the key top 4 is removed, then the stem 3 is moved upwardly back to its upper position of FIG. 5. Upon such upward movement of the stem 3, the pressing force to the movable contact element pressing portion 21 of the pressing piece 7a of the operating body 7 by the acting projection 11c of the pressing spring 11 is removed. Consequently, the thin metal plate 8b of the movable contact element 8 is spaced away from the fixed contact 10a of the fixed contact element 10, thereby bringing the push-button switch into an off state again.
There is a problem with the switch element 6 having the construction described above. After molding, a sink 23 will appear on the movable contact element pressing portion 21 where it contacts the acting projection 11c of the pressing spring 11. Referring to FIG. 9(a), the sink 23 appears because of the thickness t.sub.2 of material of the movable contact element pressing portion 21 of the pressing piece 7a. Because the thickness t.sub.2 is so great, and because the outer surfaces of the pressing piece 7a are cooled by exposure to the outside atmosphere during molding, the resin at the outer surfaces of the movable contact element pressing portion 21 of the pressing piece 7a, especially at the large surface where the movable contact element pressing portion 21 is contacted by the acting projection 11c of the pressing spring 11, cools and solidifies during molding before the resin at the inner portions of the movable contact element pressing portion 21 solidifies. This solidification upsets the flow of resin material flowing in from the leg portion 20 during molding. Resin continues to move up the leg portion 20 to the upper extremities of the pressing piece 7a, but the resistance created by the solidification of the outer surfaces results in less resin entering the movable contact element pressing portion 21 of the pressing piece 7a than desired, and this produces the sink 23.
If there is a sink 23 on the movable contact element pressing portion 21 of the pressing piece 7a where it is contacted by the acting projection 11c of the pressing spring 11, a dispersion may appear in timings at which the rib 22 on the opposite face of the movable contact element pressing portion 21 of the pressing piece 7a of the operating body 7 presses against the movable contact element 8. The sink 23 causes the dispersion. The dispersion occurs because the acting projection 11c may contact a different part of the sink 23 on the movable contact element pressing portion 21 of the pressing piece 7a on one switch operation than on the next switch operation. For example, if the acting projection 11c hits a lower part of the sink 23 on one operation than on the next, the switch will turn on later in the first operation than in the second.